Modern nuclear fuel bundles for boiling water nuclear reactors include a matrix of vertical upstanding and sealed nuclear fuel rods. These fuel bundles are held together by a lower tie plate for supporting the fuel rods and permitting the entry of water, and an upper tie plate for permitting the exit of water and generated steam. Some of the fuel rods connect to the tie plates and together with the tie plates hold the fuel rods of the fuel bundle together as a discrete unit. This fuel bundle unit is placed within a so-called channel.
In operation, the channel serves to isolate flow in the channel interior discretely through the rods of the fuel bundle. This interior flow is a characteristic particular to boiling water nuclear reactors and is away from a surrounding water filled core bypass region. The core bypass region which is filled with water provides improved moderation when the reactor is operating. Penetration of control rods into the core bypass region to displace the water and absorb neutrons occurs to control of the reaction.
The individual fuel rods are long--in the order of 160 inches--and slender. Under the dynamics of steam generation and absent any kind of restraint, these fuel rods would vibrate into abrading contact one with another. For this reason, there has developed in the art so-called fuel rod spacers.
The general construction and function of the fuel rod spacers is generally easy to understand. Specifically, a fuel spacer discretely surrounds each fuel rod at a discrete cell in a matrix of fuel rod cells within each spacer. Generally, five to nine--and usually seven--such spacers are utilized at differing elevations along the length of the fuel bundle. The result is that the fuel rods at the differing elevations along the length of the fuel bundle are held in a centered relationship.
Spacers cells surrounding each fuel rod have two primary structures acting on the fuel rods. These structures are a system of stops and springs for centering of the fuel rod. The stops define the design centered positions of the fuel rods. When the fuel rods are pushed against the stops of each spacer cell, the fuel rod is in its centered position. The springs provide the biasing force. When the fuel rods are biased by the springs into the stops of each spacer cell, centering of the fuel rod in its matrix position occurs.
Spacers can be classified by the material from which they are constructed. First, there have been spacers with Zircaloy bodies. These Zircaloy bodies define each cell as well as the individual rod stops required for each cell. The necessary springs have heretofore been Inconel springs. These Inconel springs have been held in place by surrounding parts of the Zircaloy cells.
Other spacers have been made just from Inconel. These spacer usually define a fine matrix of Inconel and contain integrally both spring and stops formed into the Inconel.
The choice between the two types of spacer construction involves balancing of both nuclear design factors for optimized nuclear reaction as well as the thermodynamic design factors for the generation of steam.
Over simplified, from the standpoint of neutron absorption, Zircaloy spacers are preferred. Simply stated., the neutron absorption cross section of Zircaloy is low compared to that of Inconel. Less neutron absorption gives greater nuclear efficiency over the life cycle of the fuel bundle.
Unfortunately, Inconel has a much higher neutron absorption cross section. Thus, the use of the Inconel springs directly detracts from the nuclear efficiency of Zircaloy spacers.
Over simplified, ferrule type spacers which surround each fuel rod are generally preferred from a thermodynamic standpoint, especially in the upper two phase region of the fuel bundle. It has been found that this more complete surround in the upper two phase region of the fuel bundle can have beneficial effects on critical power by maintaining a water layer over the surface of the steam generating fuel rods.
The ferrules of the prior art have been either round or octagonal. Where such spacers are octagonal, the respective octagons of a matrix of spacer cells surround each fuel rod. In between immediate adjacent fuel rods, the flat sides of the octagon are joined. Diagonally of the fuel rods, the flat sides of the octagon cells define a bounded flow space within a so-called "sub-channel" flow volume within the fuel bundle. Since the following invention relates to a generally octagon shaped spacer, understanding of such terminology is important.
Unfortunately, modern fuel bundle designs in boiling water nuclear reactors have added a further complicating factor. The density of the fuel rod matrix has increased. Fuel bundle matrices have increased from 8 by 8, to 9 by 9, 10 by 10, and even 11 by 11. Such increase directly impacts on the dimensions available to contain the spacer material between the fuel rods. Remembering that the cross section of the fuel bundle is a design parameter established in the original construction of the reactor, this relationship is relatively easy to understand.
Nuclear fuel rods have cylindrical Zircaloy cladding surrounding the contained nuclear fuel pellets. Taking the case of a matrix of increased density, more of the available area within the fuel bundles simply has to be occupied by the walls of the Zircaloy ferrules, which ferrules are nominally in the order of 0.020 to 0.030 inches thick. Taking the case of a 10 by 10 matrix with fuel rod diameter=0.404", center to center spacing=0.510", the gap between two adjacent fuel rods is 0.106". Two thicknesses of ferrule material uses at least 0.40" of this gap. As a result, design pressure is on spacer construction to occupy as little of the interval as possible.
Regarding this design pressure, reduction of the space occupied by the spacer between individual fuel rods to a single layer of metal is vital. Unfortunately, prior art Zircaloy ferrule spacer constructions having Inconel spring material trapped to the Zircaloy material of the spacer body have required double layers of metal within the spacer. Further, the corrosion properties of Zircaloy require that this metal be maintained in a nominal thickness in the order of about 0.020 of an inch.
Having set forth these parameters, the following summarized invention can be understood.